I have the same cracking on my '93, I think it is just metal fatigue so you would probably have to cut out the area and weld in a new thicker piece to prevent it from happening again. I can't think of a way just to add a plate to the back and prevent the cracking, you wouldn't really be able to weld it so you would have to use high strength adhesives.
But thats just my 2 cents.
J.